Twin_Turbo, What I did was "tun them on" While I was going 80 mph. I had never had any problems with when I had them on and themn went fast. I ahd an 82 and and was trying to get from San Clemente to Monterey real fast. I was traveling at least 80 when it was getting dark. Never even thought about slowing down. Well when they popped up, it was like 2 small parashutes (sp). I had travelled across the country many times. I made the best times at night.

Steve

 

Took a look at their web site, couldn't find what you were refering to... Was it called a "splash shield"?

 

It's the rear underbody panel that the Duntov Turbo's had too.

 

There's a fella here in Austin named John Tieman who open road races a 71 in the 140 to 160 classes. He has lowered the car all around with a rake to lower the nose. He has a flexible front spoiler that occasionally scrapes the pavement. He hasn't done anything special to deal with undercar turbulence since the front end pushes the air around the sides of the car. He does not run with a spare tire or spare carrier.

 

These guys have a nice rake, a rear diffuser and the front underside is almost flat w/ sheet metal:

 

Hey aharte, is was reading your post a got and idea for a wind tunnel fund raiser and started a new thread:
http://forums.corvetteforum.com/zerothread?id=575090

 

In an old issue of shark Quarterly, there's a reference to a book called "Race Car Vehicle Dynamics" by William and Douglas Milliken that covers aero plus many other topics (890 pages). Anyone have a copy?

In the same issue, there's a piece tht states that unless a rear wing is as high above the rear deck as its depth (leading edge to trailing edge dimension) it creates drag with little, if any, downforce.

 

Wow! What a GREAT discussion. Takes me back to the days when I studied fluid mechanics. That was before the availability of any computer smaller than a house and we were still using slide rules. (We could start an Off-Topic thread on what those are for you youngsters.) Made all this type of analysis very tedious. Anyway, I'm thouroughly enjoying the knowledge and ideas you fellas are sharing. It's good stuff. Thanks.

:hurray: :hurray: :hurray: :hurray: :hurray:

 

:lol: :yesnod:

 

You know I have heard the same thing in the past. But that confuses me when you see so many performance based cars "ferarri" that still use lower slim design spoilers. Then for instance, hop in a 92 or so 3000gt with an automatic spoiler, you can feel the spoiler raise, and car literally sucks about in inch lower to the ground. I took a rx7 up to about 120 and felt a odd humming, the rear tires were rubbing against the wheel well. and the front lip was skipping across the ground.When at a stand still the car still sat around 4 inches off the ground. Biggest improvement is the Audi TT, in 2000 "I believe" they had their car with no spoiler and though they look really cool that way, they kept losing control at high speeds. Audi quickly solved this problem by placing that really small lip on the rear, which creates enough down force to keep the butt planted. None of the cars mensioned have a spoiler that is above the roofline, infact most are less than half the height of the roofline. But yet I have seen/felt and read that with these suttle spoilers they were still able to create sufficent downforce.

 

Chuck- Our local S.J.S.U. has a windtunnel available :D :cheers: I work at Speedring and a few of our shop guys are taking fluid dynamics at S.J.S.U. One of them has actually gotten windtunnel time.

This is a very interesting topic, how did I know that since the topic of C3 aerodynamics came up, I'd be seeing a picture of that blue racing Stingray? One of my uncles works at Los Gatos Ferrari and you should see the underside of a Testarossa or 360 Modena. What a cleeeeeean bellypan they have :eek: As I've read above, I'd just be happy to find some way to build a fiberglass or aluminum bellypan for my '69. What's under our cars that would get in the way behind the oil pan? Exhaust? It's tucked up through the crossmember. I'm wondering about something just built from the firewall back. It looks as if a bellypan could be accomplished with side exit exhaust. Otherwise it will take some magical rerouting of the existing piping essentially tucking it up higher; but how?

I'll be editing this post; I'm going to get intimate and shoot some pictures of my car's underside.


[Modified by 1969Vette350, 3:02 PM 6/3/2003]

 

This is getting GOOD! :cheers:

Please keep me posted,

Chuck

 

I figured I might as well get a number out of this figure... The drag coeffcient works out to be between 0.39 and 0.44 depending on how you want to define the cross-sectional area and what the weather was like that day. I also assume that they mean 105 rwhp. That's the type that would make sense to quote in this situation, so I'll assume the engineer who said that was sane.

Anyway, that's not too bad, and agrees with a reference I have that gives Cd=0.36-0.38 (their error estimation) for an 82.

 

I'm pretty sure that's not going to be helpful. It appears that these people http://www.eldinc.com/user/list.htm sold SJSU their wind tunnel. Check out the cross section (search for san jose. its the 3rd match): 12 inches.

I looked up McLaren's wind tunnel at http://www.mclaren.com/technologycentre/. Its 145 meters long with a fan pulling up to 1 megawatt of electricity.

 

That's a little misleading. It would probably be more clear in context. As WashingtonRacer stated, little wings can work. The point is that their detailed design is very different if they're close the body. It acts less like an airplane wing, and as a way to reshape airflow around the body. That basically means that what the engineers have labeled the "wing equations" in their textbooks don't work for short wings, so they call them something else! Lesson: don't take a high mounted wing and shorten the mounting posts. That won't make any downforce.

The distinct wings on race cars are more efficient though. That may also have been what the author meant.

 

It looks like they've changed up their website in the last month. I bet it going to be under the Ground Effects, Exhaust Covers link which is not active yet.

Mark

 

and I quote:
"To be effective rear mounted wings must obey some basic rules of design which were first developed with early biplanes. Namely, they must be mounted at least one chord length off the body. Expressed less techncally (and in very rough terms), the wing must be mounted at least as high off the body as its own width. The "biplane rule" says that if the two wings are too close together, the low pressure on the top of the bottom wing cancels the high pressure below the top wing. In the case of an automobile, the wing must also be in clean air (like an airplane wing) to maximize its effectiveness without incurring undue drag. If a wing is too close to the body, the result is a large drag penalty with little, if any, associated downforce."
Shark Quarterly, Spring 1996 pg 34

 

Ok, I see how the guy who wrote that article is thinking about it. It seems like he didn't really know what he was talking about, but got it only partially right. Its a combination of both possibilities I mentioned above.

He's right in the sense that a separated wing with good design will have a downforce/drag ratio several times higher than a comparable spoiler (that's why they're used on race cars). He's wrong in that that doesn't mean spoilers are useless.

Think about an airplane wing. It generates lift more or less in two ways. The first is due to the shape of the wing smoothly creating a pressure differential (in various ways) between its two sides. But what if you want more or less lift for turning, climbing, or diving? You use the control surfaces. They take a little rectangle and angle it with respect to the wing. There is often still surface underneath the ailerons (ie the airflow is not "clean"), yet they're very effective at turning the plane (think how small they are). These are basically the same as functional spoilers. I say functional, because some cars do have spoilers that really just add drag.

The reason the pressure argument doesn't hold is that spoilers generate lift differently from wings. Say air is moving towards an angled spoiler. The air initially has a horizontal momentum, but the air can't go through the spoiler, so its momentum has to be changed. That directly generates a force. Unlike a normal wing, its a much simpler and more robust mechanism. Unfortunately, you can mentally tilt that picture 90 degrees to see it will also create a lot of drag.

 

Some pics I scanned in:

Graphs showing the effects of spoilers. (CL_f is the lift coefficient over the front axle and CL_R is the lift over the rear axle. CD is the drag coefficient).



It works very nicely. It even reduces the drag coefficient slightly sometimes by smoothing out flow in the rear of the car.

Graph showing a wing mounted at different heights above the body:



Just as the guy said, the downforce goes to zero as a wing is mounted closer and closer to the body. Its not that downforce can't be created there, its just that a wing is the wrong design.

 

First off, what book is that out of? At least ya gotta give credit where credit is due :D I'd love to flip through it even if it's a college manual for some Ph D course.

Okay, another thing. As mentioned before we have now gotten into control surfaces. The instant that was mentioned I thought of...flaps :thumbs: One job of flaps are to keep a plane aloft at a slower speed, they help generate lift; or if inverted would generate downforce. Do you think it would ever be prudent to have some kind of flap on a racecar? I can see it now, electronically controlled flaps built into canards fixed to F1 cars. Instead of braking by turning kinetic energy into heat energy, they'd slow down by maximizing drag for X amount of time and then return the flap to its original position.

I heard about a test Mark Donohue ran. He applied an air brake to one of his cars and tested it once. It had to have scared the crap out of him because it was never continued to be developed. I can just imagine driving along at 120 or so and pulling a lever that would raise some metal object off the car at least five times larger than our popup headlight surface area :eek: Talk about wetting yourself.


[Modified by 1969Vette350, 12:14 AM 6/4/2003]